Group III-V compounds, such as gallium nitride (GaN) and aluminum nitride (AlN), have been heretofore known as Group III nitride semiconductors. These Group III nitride semiconductor materials are utilized for configuring semiconductor light-emitting devices, such as light-emitting diodes (LEDs) and laser diodes (LDs), which emit a visible light of short wavelength in a blue color or a green color (refer, for example, to JP-B SHO 55-3834). They are also utilized for configuring electronic devices, such as Schottky junction Metal Semiconductor Field-Effect Transistors (MESFETs), that operate at a high frequency (refer, for example, to M. A. Khan et al., Applied Physics Letters (U.S.A.), 1993, Vol. 62, p. 1786).
The semiconductor devices formed of these Group III nitride semiconductor materials are configured by using a sapphire (α-Al2O3) bulk single crystal (refer, for example, to JP-A HEI 6-151963) or a cubic silicon carbide (SiC) bulk single crystal as the substrate (refer, for example, to JP-A HEI 6-326416). For example, the production of a short-wavelength visible LED by utilizing a stacked structure provided on a sapphire substrate with a clad layer formed of a Group III nitride semiconductor material and a light-emitting layer has become feasible (refer, for example, to JP-A HEI 6-151966).
The sapphire ordinarily used as the substrate for a Group III nitride semiconductor device, however, does not exhibit a fully satisfactory matching property on the crystal lattice with a Group III nitride semiconductor material, such as GaN. Thus, the use of sapphire as the substrate entails a problem that the Group III nitride semiconductor layer revealing crystalline defects, such as dislocation, only insignificantly and excelling in crystallinity is not stably obtained. When the silicon carbide bulk single crystal excelling in thermal conductivity is used as the substrate, it is convenient for the configuration of an LED excelling in the property of breakdown voltage relative to static electricity and a MESFET excelling in the property of radiating heat. The silicon carbide bulk single crystal that possesses a large aperture proper for the crystal to be utilized for the substrate, however, is expensive and proves to be disadvantageous in producing a general-purpose Group III nitride semiconductor device at a low price.
The silicon single crystal (silicon) essentially excels in thermal conductivity and has already reached the stage of allowing mass production of a single crystal of large diameter and good conductivity. When the silicon of large size and good conductivity is utilized as the substrate, therefore, it is expected to enable practical use of an inexpensive conventional LED which gives a high breakdown voltage relative to static electricity. When the silicon abounding in thermal conductivity in spite of high resistance is utilized as the substrate, it is expected to realize the MESFET fit for low-loss high-frequency telecommunication. Since the silicon single crystal has a lattice constant “a” of 0.543 nm, however, it produces a large lattice mismatch with a Group III nitride semiconductor, e.g. hexagonal GaN (a-axis lattice constant=0.319 nm). The mismatch of the silicon is still large with cubic GaN (a=0.451 nm). The silicon substrate, therefore, has been at a disadvantage in rendering difficult stable formation of a Group III nitride semiconductor layer exhibiting superior quality and revealing crystalline defects only insignificantly.
For the purpose of overcoming this disadvantage, the means of inserting during the formation of the Group III nitride semiconductor layer on the single crystal substrate having a large lattice mismatch a buffer layer intended to relax the mismatch of lattices of these two components has been generally followed heretofore. In the case of forming the Group III nitride semiconductor layer on the silicon substrate, the conventional technique of forming the Group III nitride semiconductor layer via a thin film layer of a cubic 3C-type silicon carbide (3C-SiC) has been known (refer, for example, to T. Kikuchi et al., Journal of Crystal Growth (Holland), 2005, Vol. 275, Nos. 1-2, pp. e12150-e1221). Since the quality of the 3C-SiC thin film layer conspicuously varies the crystallinity, for example, of the Group III nitride semiconductor layer formed thereon, this technique is at a disadvantage in failing to form stably the Group III nitride semiconductor layer of superior quality. Even when using a buffer layer of SiC, a Group III nitride semiconductor layer having been formed thereon cannot always have its surface excellent in flatness. This is problematic.
Single crystals excelling in electrical conductivity and in heat radiating heat and possessing a large diameter have been already in mass production. For the purpose of obtaining a semiconductor device using silicon as the substrate and excelling in optical and electrical properties, it is necessary that a buffer layer be so formed as to relax the lattice mismatch with the silicon substrate and bring about a Group III nitride semiconductor layer of superior quality. Even when the cubic SiC layer is used as the buffer layer in the case of forming the Group III nitride semiconductor layer on the silicon substrate, the SiC buffer layer is required to be so formed as to relax favorably the lattice mismatch between the two materials and allow formation as an upper layer of the Group III nitride semiconductor layer containing crystalline defects with a low density, excelling in crystallinity, and also excelling in flatness of the surface.
This invention has been produced with a view to overcoming the aforementioned problems encountered by the prior art and is aimed at providing a semiconductor device capable of using a SiC buffer layer on a single crystal substrate, forming thereon a Group III nitride semiconductor layer containing crystalline defects with a low density and excelling in crystallinity, and enhancing the light-emitting property and the high-frequency performance and a method for the production of the semiconductor device.